Electrochemical devices depend on the flow of protons, or the flow of both protons and electrons, through a proton conducting material, such as a membrane. Accordingly, materials which conduct protons, or both protons and electrons, have applications as electrolytes or electrodes in a number of electrochemical devices including fuel cells, electrochemical or supercapacitors, sensors, hydrogen separation membranes and membrane reactors.
One particularly important application for these materials is in fuel cells. Fuel cells are attractive alternatives to combustion engines for a wide variety of applications, because of their higher efficiency and the lower level of pollutants produced from their operation. There are three common types of fuel cells relevant to this patent: 1) a direct hydrogen/air fuel cell system, which stores hydrogen and then delivers it to the fuel cell as needed; 2) an indirect hydrogen/air fuel cell, in which hydrogen is generated on site from a hydrocarbon fuel, cleaned of carbon monoxide, and subsequently fed to the fuel cell; and 3) a direct methanol fuel cell (“DMFC”), which feeds a methanol/water solution directly to the fuel cell. An example of this later fuel cell was described, for example, in U.S. Pat. No. 5,559,638, the disclosure of which is incorporated herein by reference.
Regardless of the fuel cell design chosen, the operating efficiency of the device is limited by the efficiency of the electrolyte at transporting protons. Typically, perflourinated sulphonic acid polymers, polyhydrocarbon sulfonic polymers, and composites thereof are used as electrolyte membrane materials for fuel cell. However, these conventional materials utilize hydronium ions (H3O+) to facilitate proton conduction. Accordingly, these materials must be hydrated, and a loss of water immediately results in degradation of the conductivity of the electrolyte and therefore the efficiency of the fuel cell. Moreover, this degradation is irreversible, i.e., a simple reintroduction of water to the system does not restore the conductivity of the electrolyte.
As a result, fuel cells utilizing these materials require peripheral systems to ensure water recirculation and temperature control to keep the water from evaporating. Not only do these systems increase the complexity and cost of these fuel cells, but because the system cannot exceed a temperature of 100° C. the fuel cell catalysts and other systems cannot be operated a maximum efficiency. Higher temperatures would also reduce carbon monoxide poisoning of the fuel cell catalyst.
It has recently been shown that the solid acids such as CsHSO4 can be used as the electrolyte in fuel cells operated at temperatures of 140-160° C. Use of this material greatly simplifies fuel cell design relative to polymer electrolyte fuel cells because hydration of the electrolyte is not necessary and, because of the elevated temperature of operation, residual CO in the fuel stream can be better tolerated. The high conductivity of CsHSO4 and analogous materials results from a structural phase transition that occurs at 141° C. from an ordered structure, based on chains of SO4 groups linked by well-defined hydrogen bonds, to a disordered structure in which SO4 groups freely reorient and easily pass protons between one another. Thus, disorder in the crystal structure is a key prerequisite for high proton conductivity.
Ultimately, solid acid electrolytes may solve many of the problems facing state-of-the-art polymer based fuel cells. These problems include inability to operate at temperatures above 100° C. (which would increase the CO tolerance of the Pt catalyst), humidification requirements, and methanol permeation across the electrolyte. The technological objectives of this work are thus to simply fuel cell operation by use of alternative electrolytes. However, the lifetime of these sulfate and selenium based solid acids is insufficient for commercial applications. The poor lifetime of both CsHSO4 and CsHSeO4 under fuel cell operating conditions results from the reduction of sulfur and selenium by hydrogen in the presence of typical fuel cell catalysts, according to:2CsHSO4+4H2 - - - >Cs2SO4+H2S+4H2O.2CsHSeO4+4H2 - - - >Cs2SeO4+H2Se+4H2O.Accordingly, a need exists for solid acid compounds with high proton conductivity that are stable under fuel cell conditions, and processing methodologies that lead to high performance membrane-electrode-assemblies (MEAs) based on these solid acid compounds.